1. Field of the Invention
The present invention relates to a new and improved fuel for internal combustion engines employing electrical fuel ignition and to the use of methyl formate as a fuel additive.
2. Discussion of the Background and Material Information
As is generally known in this technology, the efficiency of thermal engines increases the greater the temperature difference between the incoming medium and the outgoing medium. The consequence of this phenomenon in the case of internal combustion engines is that there must be simultaneously employed increasingly greater pressures in order to achieve an increased working or operating temperature. In the case of internal combustion engines with positively induced ignition of the fuel mixture this fuel mixture must not be self-igniting. As a measure of this capability there has been introduced the octane number. Depending upon the differently employed methods of determining such octane number, reference is made to the research octane number (RON), to the motor or engine octane number (MON), the street or road octane number (SON), and the front octane number (FON). Since the RON is determined both under lesser mechanical as well as thermal loads than the MON, a great many fuels have a lower RON than MON. However, the MON constitutes a value which more closely reflects practice since it is determined under more stringent conditions. In the case of the FON the procedure for determining the same entails taking a fraction from the fuel, which fraction distills over up to 100.degree. C., and from such fraction there is then determined the RON. Therefore, the FON is a measure of the knock rating of the fuel constituents which boil at the start.
Since a difference is present, especially when there are added anti-knock agents, between the real behavior of the fuel in an engine and the determined values, such as MON, RON and FON, there are additionally performed road tests which are carried out with mass produced engines. In that case reference is made to the SON. These tests are of particular significance inasmuch as it has been found that, for instance, the addition of tetra ethyl lead during road tests has proven to be more effective than with the test engines which have been used for determining the RON or the MON or the FON. In this connection, reference is made to a so-called lead bonus.
With the employment of catalysts in catalytic converters for the further catalytic chemical conversion of the engine exhaust gases, as such has been initiated in the United States, it was necessary to develop knock-resistant, lead-free fuels, since lead acts as a catalyst poison and therefore would correspondingly impair the effectiveness of the catalysts of the catalytic converters.
Instead of using lead compounds there can be used, for example, also other iron or manganese compounds as anti-knock agents. These compounds exhibit high toxicity, and furthermore, oxides remain in the combustion compartment which, to the extent that no other additives are provided in the fuel, can lead, on the one hand, to premature wear of the pistons and cylinders of the internal combustion engine and, on the other hand, to a premature ignition of the fuel mixture by incandescent residues. This phenomenon is known in publications as "post dieseling". In the case of an engine which is exposed to increased loads, this phenomenon can even result in melting of the pistons.
Owing to the use of catalysts in catalytic converters it is necessary to provide lead-free fuels and since, furthermore, for health reasons attempts have been made to maintain lead emissions as low as possible, the lead content of fuels has been systematically lowered or even totally eliminated, and there are employed other anti-knock agents. Methyl tert.-butyl ether should be here mentioned as an anti-knock agent which has found particularly widespread use in more recent times. This compound has a boiling point of 55.3.degree. C. and a density of 0.7458 g/cm.sup.3 at 15.degree. C. With the addition of methyl tert.-butyl ether (MTBE), depending upon the composition of the basic gasoline, there can be attained an RON between 115 and 135 and an MON between 98 and 120. The addition of MTBE is accomplished within the limits of 3.0 percent by volume and 15.0 percent by volume of the gasoline. What is disadvantageous with the addition of MTBE is that the increase of that measuring number which is significant for the normal operation of an engine, namely, the MON, is not effected to that desired extent as in the case of the RON.
Apart from the addition of anti-knock agents a large number of further substances are added to fuels in order to obtain a certain fuel property level. Therefore, fuel additives are known for avoiding fouling of the carburetor. Also, there are known additives for retarding fuel oxidation, in order to avoid the formation of resin-like tacky residues. Further additives are those which are intended to prevent the corrosion of the metals by the fuel. Other additives are those forming copper complexes in order to prevent oxidation of the fuel and also additives intended to prevent icing of the carburetor. Regarding the group of additives used to prevent carburetor icing, there are either employed surfactants or compounds which lower the freezing point of water. As to such multiplicity of additives there are mentioned in the literature amines, diamines, amides, ammonium salts of diesters of phosphoric acids, glycerine, alcohols, glycols, ketones, dimethylformamide and dimethylacetamide.
In German Published Patent Application No. 2,447,345, published Apr. 15, 1976, there have been disclosed synthetic fuel mixtures composed of methanol, formaldehyde dimethylacetal and formic acid methyl ester, which are intended to be used as synthetic fuel mixtures. In order to obtain an appropriate octane number, iron carbonyl and organic manganese compounds are provided as additives.